diff --git a/Sources/Stag/CaptureManager.swift b/Sources/Stag/CaptureManager.swift
index 7728906..d2605d0 100644
--- a/Sources/Stag/CaptureManager.swift
+++ b/Sources/Stag/CaptureManager.swift
@@ -393,10 +393,7 @@ final class CaptureManager {
     }
 
     private func saveJPEG(_ image: NSImage, to url: URL, quality: Double) {
-        guard let tiff = image.tiffRepresentation,
-              let bitmap = NSBitmapImageRep(data: tiff) else { return }
-        let props: [NSBitmapImageRep.PropertyKey: Any] = [.compressionFactor: quality]
-        guard let data = bitmap.representation(using: .jpeg, properties: props) else { return }
+        guard let data = image.encoded(as: .jpeg(quality: quality)) else { return }
         try? data.write(to: url)
     }
 
diff --git a/Sources/Stag/Models/CaptureHistoryStore.swift b/Sources/Stag/Models/CaptureHistoryStore.swift
index 3f16296..ca35eeb 100644
--- a/Sources/Stag/Models/CaptureHistoryStore.swift
+++ b/Sources/Stag/Models/CaptureHistoryStore.swift
@@ -140,20 +140,14 @@ final class CaptureHistoryStore: ObservableObject {
     }
 
     static func writeThumbnail(_ image: NSImage, to url: URL) {
-        let maxDim: CGFloat = 320
-        let w = image.size.width, h = image.size.height
-        guard w > 0, h > 0 else { return }
-        let scale = min(maxDim / w, maxDim / h, 1.0)
-        let thumbSize = CGSize(width: w * scale, height: h * scale)
+        let thumbSize = ThumbnailGeometry.fittedSize(for: image.size, maxDimension: 320)
+        guard thumbSize.width > 0 else { return }
         let thumb = NSImage(size: thumbSize)
         thumb.lockFocus()
         image.draw(in: NSRect(origin: .zero, size: thumbSize),
                    from: NSRect(origin: .zero, size: image.size), operation: .copy, fraction: 1)
         thumb.unlockFocus()
-        let props: [NSBitmapImageRep.PropertyKey: Any] = [.compressionFactor: 0.7]
-        guard let tiff = thumb.tiffRepresentation,
-              let bm = NSBitmapImageRep(data: tiff),
-              let data = bm.representation(using: .jpeg, properties: props) else { return }
+        guard let data = thumb.encoded(as: .jpeg(quality: 0.7)) else { return }
         try? data.write(to: url)
     }
 
diff --git a/Sources/Stag/Utils/ImageExportFormat.swift b/Sources/Stag/Utils/ImageExportFormat.swift
new file mode 100644
index 0000000..1a27724
--- /dev/null
+++ b/Sources/Stag/Utils/ImageExportFormat.swift
@@ -0,0 +1,34 @@
+import Cocoa
+
+/// The on-disk encoding for an exported image. Centralizes the format choice and
+/// the encode-to-`Data` step that were previously inlined (and, for JPEG,
+/// duplicated) across the editor's save path and CaptureManager.
+enum ImageExportFormat: Equatable {
+    case png
+    case jpeg(quality: Double)
+
+    /// Chooses a format from a destination path's extension: `.jpg`/`.jpeg` →
+    /// JPEG (quality 0.9), anything else → PNG.
+    static func forPath(_ path: String) -> ImageExportFormat {
+        let lower = path.lowercased()
+        if lower.hasSuffix(".jpg") || lower.hasSuffix(".jpeg") {
+            return .jpeg(quality: 0.9)
+        }
+        return .png
+    }
+}
+
+extension NSImage {
+    /// Encodes the image to `Data` in the given format, or `nil` if encoding fails.
+    func encoded(as format: ImageExportFormat) -> Data? {
+        switch format {
+        case .png:
+            return pngData
+        case .jpeg(let quality):
+            let props: [NSBitmapImageRep.PropertyKey: Any] = [.compressionFactor: quality]
+            return tiffRepresentation
+                .flatMap { NSBitmapImageRep(data: $0) }
+                .flatMap { $0.representation(using: .jpeg, properties: props) }
+        }
+    }
+}
diff --git a/Sources/Stag/Views/Editor/EditorView.swift b/Sources/Stag/Views/Editor/EditorView.swift
index 9b4c5d7..334eef9 100644
--- a/Sources/Stag/Views/Editor/EditorView.swift
+++ b/Sources/Stag/Views/Editor/EditorView.swift
@@ -1871,17 +1871,7 @@ struct EditorView: View {
     @discardableResult
     private static func writeImageSync(_ image: NSImage, to path: String) -> Bool {
         let url = URL(fileURLWithPath: path)
-        let lower = path.lowercased()
-        let data: Data?
-        if lower.hasSuffix(".jpg") || lower.hasSuffix(".jpeg") {
-            let props: [NSBitmapImageRep.PropertyKey: Any] = [.compressionFactor: 0.9]
-            data = image.tiffRepresentation
-                .flatMap { NSBitmapImageRep(data: $0) }
-                .flatMap { $0.representation(using: .jpeg, properties: props) }
-        } else {
-            data = image.pngData
-        }
-        guard let data else { return false }
+        guard let data = image.encoded(as: .forPath(path)) else { return false }
         do {
             try FileManager.default.createDirectory(at: url.deletingLastPathComponent(),
                                                     withIntermediateDirectories: true)
diff --git a/Tests/StagTests/ImageExportFormatTests.swift b/Tests/StagTests/ImageExportFormatTests.swift
new file mode 100644
index 0000000..a9ffbb2
--- /dev/null
+++ b/Tests/StagTests/ImageExportFormatTests.swift
@@ -0,0 +1,56 @@
+import XCTest
+import Cocoa
+@testable import Stag
+
+/// Format selection + encoding extracted from EditorView.writeImageSync and
+/// unified with CaptureManager.saveJPEG.
+final class ImageExportFormatTests: XCTestCase {
+
+    // MARK: forPath
+
+    func testPNGExtension() {
+        XCTAssertEqual(ImageExportFormat.forPath("/tmp/shot.png"), .png)
+    }
+
+    func testJPGExtensionIsJPEGAt90() {
+        XCTAssertEqual(ImageExportFormat.forPath("/tmp/shot.jpg"), .jpeg(quality: 0.9))
+    }
+
+    func testJPEGExtension() {
+        XCTAssertEqual(ImageExportFormat.forPath("/tmp/shot.jpeg"), .jpeg(quality: 0.9))
+    }
+
+    func testExtensionMatchIsCaseInsensitive() {
+        XCTAssertEqual(ImageExportFormat.forPath("/tmp/SHOT.JPG"), .jpeg(quality: 0.9))
+        XCTAssertEqual(ImageExportFormat.forPath("/tmp/SHOT.PNG"), .png)
+    }
+
+    func testUnknownOrMissingExtensionDefaultsToPNG() {
+        XCTAssertEqual(ImageExportFormat.forPath("/tmp/shot"), .png)
+        XCTAssertEqual(ImageExportFormat.forPath("/tmp/shot.gif"), .png)
+        XCTAssertEqual(ImageExportFormat.forPath("/tmp/shot.tiff"), .png)
+    }
+
+    // MARK: encoded(as:)
+
+    private func solidImage() -> NSImage {
+        let image = NSImage(size: NSSize(width: 4, height: 4))
+        image.lockFocus()
+        NSColor.red.setFill()
+        NSRect(x: 0, y: 0, width: 4, height: 4).fill()
+        image.unlockFocus()
+        return image
+    }
+
+    func testEncodePNGHasPNGSignature() {
+        let data = solidImage().encoded(as: .png)
+        XCTAssertNotNil(data)
+        XCTAssertEqual(Array(data!.prefix(4)), [0x89, 0x50, 0x4E, 0x47])  // \x89PNG
+    }
+
+    func testEncodeJPEGHasJPEGSignature() {
+        let data = solidImage().encoded(as: .jpeg(quality: 0.8))
+        XCTAssertNotNil(data)
+        XCTAssertEqual(Array(data!.prefix(3)), [0xFF, 0xD8, 0xFF])  // SOI marker
+    }
+}